This invention relates generally to devices for detecting relative motion between two bodies and more particularly to transducers in which magnetoresistive devices are used to sense bidirectional motion.
Magnetoresistive devices whose resistance increases in the presence of magnetic flux are well known and frequently used as displacement sensing devices, contactless switches, variable resistors, or other control elements. For the detection of relative motion, the transducers are usually formed as pairs of serpentine lengths of magnetoresistive material connected across a potential source with a center-tap serving as an output terminal. The two elements lie in a common plane and serially encounter a magnetic flux field that varies in intensity with change in position. The voltage signal at the center-tap provides a fluctuating signal representing the density of flux encountered by each transducer element. This signal is applied to threshold detectors which can be set to respond at various signal levels.
The usual arrangements are particularly well adapted to generate a series of signals representing progression along a toothed or slotted member and are effective in either forward or reverse directions.
In typical displacement sensing configurations, the gap between the magnetoresistors and relatively moving magnetic member will vary thus creating significant changes in the output signal amplitude. The use of elements in pairs with the center-tap increases signal amplitude and compensates for temperature drift. The arrangement also readily lends itself to connection in a bridge sensing circuit. Additionally, the arrangement enables position sensing during standstill and provides signals having fast rise times with which level detectors are more sensitive. Examples of known arrangements are shown in U.S. Pat. Nos., 3,846,697; 4,021,728; 4,039,936; and 4,079,360. Additional description of magnetoresistive devices is found in "Galvanomagnetic Devices" Data Book B1645.101, 1976/77, by Siemens AG, Munich, Germany.
The known arrangements, however, have a deficiency when used to accurately indicate a particular location such as a "home" position. Because of the change in polarity of the signal with change in direction of relative motion, supplemental circuits must be used to select the desired signal. In addition, the prior circuits do not provide a pair of zero cross-overs needed for reliable triggering.